Embodiments of the invention relate generally to electrical distribution systems, and more particularly to electrical distribution systems implementing micro -electromechanical system based switching (MEMS) devices.
To protect against fire and equipment damage, electrical equipment and wiring must be protected from conditions that result in current levels above their ratings. Electrical distribution systems employ protective devices to operate (open the electrical circuit) in case of such an over-current condition. A typical electrical distribution system includes protective devices that can be found in residential, commercial, & industrial applications. Electrical distribution systems form a tree-like structure with a main incoming power (trunk) feeding ever smaller and smaller distribution lines (branches). Typically, the distribution branches break the power into smaller lines that step-down the voltage with a transformer and distribute the power to the load circuits.
Due to the enormous costs associated with a power outage (downtime, productivity loss, critical system loss, for example), it may be of interest in some applications for the system to stay online at all times unless other conditions determine otherwise. Therefore, the protection devices should operate (take power offline) under such circumstances where an over-current vault may result in an undesirable outcome is present on the distribution line, in addition, when a fault (especially a short circuit fault) occurs, it is desirable for the first and only the first protection device upstream of the fault to operate; a system in which only the closest protection device upstream of the fault trips is said to be selectively coordinated. A coordinated system serves to ensure that only the necessary equipment is taken offline during a failure and thus minimises the costs or power outages. For instance, if a fault occurs at a load and the system is selective, then only the adjacent protective device should operate; leaving all other load circuits unaffected by the fault. If the system is not selective, the distribution branch protective device, or even the main power input device, might operate taking all the loads downstream offline unnecessarily.
Electrical systems presently use either a fuse or a circuit breaker to perform over-current protection. Fuses rely on heating effects (1^2*t) to operate. They are designed as weak points in the circuit and each successive fuse closer to the load must be rated for smaller and smaller currents. In a short circuit, condition all upstream fuses see the same heating energy and the weakest one, by design the closest to the fault will be the first to operate. Fuses however are one-time devices and must be replaced alter a fault occurs. Circuit breakers on the other hand can be reset. However, to protect against a short circuit fault, some types of circuit breakers employ electromagnetic trip devices. These electromagnetic trip devices rely on the current level present and not on heating effects to trip the circuit breaker. The quick reaction to large currents makes it difficult to have a selective protection scheme with circuit breakers, which may result in increased complexity of a circuit breaker for use in such applications.
Accordingly, there exists a need in the art for a systems and methods for current limiting to provide selectively coordinated protection for electrical distribution systems.